In the field of medical technology there is a problem in producing physical measurements representing the activity in an individual's autonomous nervous system, i.e. in the part of the nervous system, which is beyond the control of the will. Particularly, there is a special need to monitor the autonomous nervous system of a sedated, non-verbal patient, e.g. a patient in anaesthesia or an artificially ventilated patient, in order to detect if the patient needs more analgesics due to pain/discomfort stimuli or hypnotics because of awakening stimuli.
Analgesics are given to avoid pain/discomfort, and hypnotics are given to avoid awakening. Pain/discomfort can induce awakening and awakening rarely induce pain/discomfort. If hypnotics is given to a patient that feels pain/discomfort, the stress activation may be reduced, but the patient may still feel pain/discomfort. It is therefore a need for providing a monitoring system that can give information about if the stress activation found in sedated patients is due to pain/discomfort stimuli or awakening stimuli.
Both patients in anaesthesia and patients that are artificially ventilated are treated with analgesics and hypnotics. Currently, the stress activation of these patients is monitored by an increase in blood pressure and heart rate. Blood pressure and heart rate is influenced by many other factors than the need of analgesics or hypnotics, like blood circulatory changes found in heart disease, hypertension, lung disease, anaemia, blood loss and sepsis to name a few.
1-2% of the patients in anaesthesia feel pain during surgery. The development in medication in anaesthesia is to give both hypnotics and analgesics with very short half-life. Then it will be even more important to monitor the patient's need of analgesics and hypnotics.
Tests have shown that the skin's conductance changes as a time variable signal which, in addition to a basal, slowly varying value (the so-called basal level or the average conductance level through a certain interval), also has a component consisting of spontaneous waves or fluctuations, in which characteristics of these fluctuations, such as for example their frequency and amplitude, are factors which are correlated with the experience of pain in the target object (the patient). Measuring and analyzing characteristics of these fluctuations is a known method of providing information concerning the activity in the sympathetic nervous system, including the effect of pain.